/*
 * Copyright 2002-2018 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      https://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.springframework.web.servlet.resource;

import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;
import org.springframework.core.io.Resource;
import org.springframework.util.FileCopyUtils;
import org.springframework.util.StringUtils;

import javax.servlet.http.HttpServletRequest;
import java.io.IOException;
import java.io.StringWriter;
import java.nio.charset.Charset;
import java.nio.charset.StandardCharsets;
import java.util.ArrayList;
import java.util.List;
import java.util.SortedSet;
import java.util.TreeSet;

/**
 * A {@link ResourceTransformer} implementation that modifies links in a CSS
 * file to match the public URL paths that should be exposed to clients (e.g.
 * with an MD5 content-based hash inserted in the URL).
 *
 * <p>The implementation looks for links in CSS {@code @import} statements and
 * also inside CSS {@code url()} functions. All links are then passed through the
 * {@link ResourceResolverChain} and resolved relative to the location of the
 * containing CSS file. If successfully resolved, the link is modified, otherwise
 * the original link is preserved.
 *
 * @author Rossen Stoyanchev
 * @since 4.1
 */
public class CssLinkResourceTransformer extends ResourceTransformerSupport {

    private static final Charset DEFAULT_CHARSET = StandardCharsets.UTF_8;

    private static final Log logger = LogFactory.getLog(CssLinkResourceTransformer.class);

    private final List<LinkParser> linkParsers = new ArrayList<>(2);


    public CssLinkResourceTransformer() {
        this.linkParsers.add(new ImportStatementLinkParser());
        this.linkParsers.add(new UrlFunctionLinkParser());
    }


    @SuppressWarnings("deprecation")
    @Override
    public Resource transform(HttpServletRequest request, Resource resource, ResourceTransformerChain transformerChain)
            throws IOException {

        resource = transformerChain.transform(request, resource);

        String filename = resource.getFilename();
        if (!"css".equals(StringUtils.getFilenameExtension(filename)) ||
                resource instanceof EncodedResourceResolver.EncodedResource ||
                resource instanceof GzipResourceResolver.GzippedResource) {
            return resource;
        }

        byte[] bytes = FileCopyUtils.copyToByteArray(resource.getInputStream());
        String content = new String(bytes, DEFAULT_CHARSET);

        SortedSet<ContentChunkInfo> links = new TreeSet<>();
        for (LinkParser parser : this.linkParsers) {
            parser.parse(content, links);
        }

        if (links.isEmpty()) {
            return resource;
        }

        int index = 0;
        StringWriter writer = new StringWriter();
        for (ContentChunkInfo linkContentChunkInfo : links) {
            writer.write(content.substring(index, linkContentChunkInfo.getStart()));
            String link = content.substring(linkContentChunkInfo.getStart(), linkContentChunkInfo.getEnd());
            String newLink = null;
            if (!hasScheme(link)) {
                String absolutePath = toAbsolutePath(link, request);
                newLink = resolveUrlPath(absolutePath, request, resource, transformerChain);
            }
            writer.write(newLink != null ? newLink : link);
            index = linkContentChunkInfo.getEnd();
        }
        writer.write(content.substring(index));

        return new TransformedResource(resource, writer.toString().getBytes(DEFAULT_CHARSET));
    }

    private boolean hasScheme(String link) {
        int schemeIndex = link.indexOf(':');
        return ((schemeIndex > 0 && !link.substring(0, schemeIndex).contains("/")) || link.indexOf("//") == 0);
    }


    /**
     * Extract content chunks that represent links.
     */
    @FunctionalInterface
    protected interface LinkParser {

        void parse(String content, SortedSet<ContentChunkInfo> result);

    }


    /**
     * Abstract base class for {@link LinkParser} implementations.
     */
    protected abstract static class AbstractLinkParser implements LinkParser {

        /**
         * Return the keyword to use to search for links, e.g. "@import", "url("
         */
        protected abstract String getKeyword();

        @Override
        public void parse(String content, SortedSet<ContentChunkInfo> result) {
            int position = 0;
            while (true) {
                position = content.indexOf(getKeyword(), position);
                if (position == -1) {
                    return;
                }
                position += getKeyword().length();
                while (Character.isWhitespace(content.charAt(position))) {
                    position++;
                }
                if (content.charAt(position) == '\'') {
                    position = extractLink(position, "'", content, result);
                } else if (content.charAt(position) == '"') {
                    position = extractLink(position, "\"", content, result);
                } else {
                    position = extractLink(position, content, result);

                }
            }
        }

        protected int extractLink(int index, String endKey, String content, SortedSet<ContentChunkInfo> linksToAdd) {
            int start = index + 1;
            int end = content.indexOf(endKey, start);
            linksToAdd.add(new ContentChunkInfo(start, end));
            return end + endKey.length();
        }

        /**
         * Invoked after a keyword match, after whitespaces removed, and when
         * the next char is neither a single nor double quote.
         */
        protected abstract int extractLink(int index, String content, SortedSet<ContentChunkInfo> linksToAdd);
    }


    private static class ImportStatementLinkParser extends AbstractLinkParser {

        @Override
        protected String getKeyword() {
            return "@import";
        }

        @Override
        protected int extractLink(int index, String content, SortedSet<ContentChunkInfo> linksToAdd) {
            if (content.substring(index, index + 4).equals("url(")) {
                // Ignore, UrlLinkParser will take care
            } else if (logger.isTraceEnabled()) {
                logger.trace("Unexpected syntax for @import link at index " + index);
            }
            return index;
        }
    }


    private static class UrlFunctionLinkParser extends AbstractLinkParser {

        @Override
        protected String getKeyword() {
            return "url(";
        }

        @Override
        protected int extractLink(int index, String content, SortedSet<ContentChunkInfo> linksToAdd) {
            // A url() function without unquoted
            return extractLink(index - 1, ")", content, linksToAdd);
        }
    }


    private static class ContentChunkInfo implements Comparable<ContentChunkInfo> {

        private final int start;

        private final int end;

        ContentChunkInfo(int start, int end) {
            this.start = start;
            this.end = end;
        }

        public int getStart() {
            return this.start;
        }

        public int getEnd() {
            return this.end;
        }

        @Override
        public int compareTo(ContentChunkInfo other) {
            return Integer.compare(this.start, other.start);
        }

        @Override
        public boolean equals(Object other) {
            if (this == other) {
                return true;
            }
            if (!(other instanceof ContentChunkInfo)) {
                return false;
            }
            ContentChunkInfo otherCci = (ContentChunkInfo) other;
            return (this.start == otherCci.start && this.end == otherCci.end);
        }

        @Override
        public int hashCode() {
            return this.start * 31 + this.end;
        }
    }

}
